Make joint state publisher's topic configurable

[chapulina]

Desired behavior

The JointStatePublisher plugin always publishes on the /world/<world_name>/model/<model_name>/state topic. It would be convenient to let users change it, especially so that models can be more easily reused across worlds of different names.

Alternatives considered

N/A

Implementation suggestion

The topic should be configurable from SDF.

The same way that we accept a joint_name element here:

https://github.com/ignitionrobotics/ign-gazebo/blob/59ff8532d0699f18cf9d0e34aa1bc0afecb73671/src/systems/joint_state_publisher/JointStatePublisher.cc#L61

We could accept a topic element, like how this other plugin does it for example:

https://github.com/ignitionrobotics/ign-gazebo/blob/59ff8532d0699f18cf9d0e34aa1bc0afecb73671/src/systems/detachable_joint/DetachableJoint.cc#L99-L106

[jasmeet0915]

Hi, I was looking for first time issues and have been able to resolve this one. I would like to make a PR for the same!

[chapulina]

That's great, thanks @jasmeet0915 !